Heretofore, ceramic heaters used for heating wafers have been known. Such ceramic heaters are required to have uniform heating properties so that the heaters can heat wafers uniformly. For example, Patent Document 1 discloses a ceramic heater including a resistive heating element composed of molybdenum and buried in a ceramic plate composed of aluminum nitride, and an aluminum nitride shaft joined to the plate, wherein the amounts of metal carbides in the resistive heating element are reduced to reduce the non-uniformity in the amounts of carbides among positions of the resistive heating element and to thereby decrease the temperature distribution in the heating surface.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-288975.
According to the ceramic heater equipped with a shaft described above, deterioration of the uniform heating property sometimes occurs in a temperature range higher than the designed temperature although the uniformity of the temperature distribution, i.e., the uniform heating property, is good near the designed temperature. For example, when the resistive heating element is heated so that the ceramic heater has a temperature higher than the designed temperature, a hot spot is generated near the center of the heating surface of the ceramic plate, thereby widening the temperature difference between the center and the periphery and deteriorating the uniform heating property. Since the uniform heating property is deteriorated at temperatures other than the designed temperature, ceramic heaters having different designed temperatures are designed every time different process temperatures are used in wafer etching, CVD, etc. However, in recent years, there has arisen a need to change temperature during a process and a heater having a uniform heating property that does not easily degrade despite the temperature changes is desired.